Irish diplomats were warned by a senior British official during critical Anglo-Irish Agreement talks that all their communications were being bugged.
The revelation came in a secret 1985 document deemed so sensitive it was sent from London to Dublin by courier to be hand-delivered to Taoiseach Dr Garret FitzGerald, Foreign Affairs Minister Peter Barry and Tánaiste Dick Spring.
The document, dated March 27 1985, was sent from Richard Ryan, a senior official based at the Irish Embassy in London.
Ironically, just two years later Dr FitzGerald publicly moved to dismiss reports of an attempt to 'bug' the Irish Embassy in London.
Mr Ryan had lunch with Lord Gowrie - now Grey Ruthven, 2nd Earl of Gowrie - in early 1985 before the Anglo Irish Agreement was signed.
Lord Gowrie, despite being a Scottish Peer, was born in Ireland and served under Prime Minister Margaret Thatcher first as Minister for Employment, then Minister for Northern Ireland and finally as Chancellor of the Duchy of Lancaster.
"(Lord) Gowrie feels honoured that the Prime Minister has asked him to join the Cabinet sub-committee which has just been set up," Mr Ryan wrote.
"He feels he must proceed with great care in order not to compromise in any way the objectivity that is essential to the job," he explained.
"He asked that the Ambassador and I should be very aware of blanket eaves-dropping in our communications with Dublin and that we should not allow our special relationship with him (known to the Prime Minister and one of the reasons for him being brought in at this time) to lead to difficulties."